Separation of suspended solids is one of the most important operations in a wastewater treatment plant (e.g. clarifiers). The range in solid sizes and their topology, usually found in wastewater, has led to a diverse set of separation approaches, which, as a consequence, also leeds to a diverse set of simulation methods when using CFD to tackle this subject (Wicklein et al, 2016). The basic distinction between methods lies on how the suspended elements are considered: either as a continuum (Eulerian approach) or as a discrete field (Lagrangian approach). The former essentially implies a mixture of several components – forcibly including water and at least one type of suspended solids – whereas in the latter each particle is tracked individually. Both have limitations regarding the type of physical processes that can be accurately modelled and simulation cost are usually high. However, choosing one over the other approach may be based on the concentration of the suspended solids: for low values Lagrangian is possible, as demonstrated by the authors at STAR Global Conference in 2016, whereas for high values an Eulerian approach is advisable. The latter is then the subject matter of this work, where we detail the implementation of a settling velocity model that allows the sedimentation of the sludge (i.e., the mixture of water and suspended solids) to occur. The impact of solid concentration is considered on both density and viscosity. Finally, results are shown and compared on standard test cases available in the literature. This particular modelling feature has been historically absent from STAR-CCM+® capabilities and, to the authors knowledge, this is the first instance where it is used on such a context.